Accelerators and ion sources
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Accelerators and Ion Sources. CHARMS Basic Physics Topics series November 2 nd , 2005. Outline. Accelerators Ion Sources (This is logically reverse order, but it is easier to present things this way). Accelerators – basic ideas. Charged particles can be accelerated in the electric field.

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Accelerators and Ion Sources

CHARMS Basic Physics Topics series

November 2nd, 2005


Outline

  • Accelerators

  • Ion Sources

    (This is logically reverse order, but it is easier to present things this way)

Accelerators and Ion Sources


Accelerators – basic ideas

  • Charged particles can be accelerated in the electric field.

  • Examples from the nature – electrostatic discharge, α- and β-decays, cosmic rays.

  • Rutherford's experiments with α-particles

    • Discovery of the nucleus in 1911

    • First artificial nuclear reactions

    • Inspiration for high-voltage particle accelerators

  • Muons and pions were discovered in cosmic-ray experiments with emulsions.

  • Everyday life: TV-set, X-ray tubes...

Accelerators and Ion Sources


Types of Accelerators Used in Science

  • Electrostatic: Cockroft-Walton, Van de Graaff

  • Induction: Induction linac, betatron

  • Radio-frequency accelerators: LINAC, RFQ, Cyclotron, Isochronous cyclotron, Synchrocyclotron, Microtron, Synchrotron

Accelerators and Ion Sources


Cockroft-Walton

  • High voltage source using rectifier units

  • Voltage multiplier ladder allows reaching up to ~1MeV (sparking).

  • First nuclear transmutation reaction achieved in 1932: p + 7Li → 2·4He

  • CW was widely used as injector until the invention of RFQ

Fermilab 750 kV C-W preaccelerator

Accelerators and Ion Sources


Van de Graaff

  • Voltage buildup by mechanical transport of charge using a conveyor belt.

  • Builds up to ~20 MV

Accelerators and Ion Sources


Tandem Van de Graaff

  • Negative ions accelerated towards a positive HV terminal, then stripped of electrons and accelerated again away from it, doubling the energy.

  • Negative ion source required!

  • Examples:

    • VIVITRON @ IReS Strasbourg

    • 25 MV Tandem @ ORNL

    • 18 MV Tandem @ JAERI

    • 20 MV Tandem in Buenos Aires

Accelerators and Ion Sources


Induction linac

  • Creation of electric field by magnetic induction in a longitudinal evacuated cavity in magnetic material

  • Very high intensity beams (up to thousands of Amperes)

N. C. Christofilos et al., Rev. of Sci. Inst. 35 (1964) 886

Accelerators and Ion Sources


Betatron

  • Changes in the magnetic flux enclosed by the circular beam path induce a voltage along the path.

  • Name derived from its use to accelerate electrons

  • To the left: Donald Kerst with two of the first operational betatrons (2.3 and 25 MeV)

Accelerators and Ion Sources


RF Accelerators

  • High voltage gaps are very difficult to maintain

  • Solution: Make the particles pass through the voltage gap many times!

  • First proposed by G. Ising in 1925

  • First realization by R. Wiederöe in 1928 to produce 50 kV potassium ions

  • Many different types

Accelerators and Ion Sources


RF LINAC – basic idea

  • Particles accelerated between the cavities

  • Cavity length increases to match the increasing speed of the particles

  • EM radiation power P = ωrfCVrf2 –

    • the drift tube placed in a cavity so that the EM energy is stored.

    • Resonant frequency of the cavity tuned to that of the accelerating field

Accelerators and Ion Sources


RF LINAC – phase focusing

  • E. M. McMillan – V. Veksler 1945

  • The field is synchronized so that the slower particles get more acceleration

Accelerators and Ion Sources


LINAC – Examples

  • SLAC – 3 km, 50 GeV electrons, 2.856 GHz

  • UNILAC @ GSI – HI

  • GELINA @ IRMM Geel – 150 MeV electrons

GELINA maquette

Accelerators and Ion Sources


RF Quadrupole

  • Simultaneous generation of a longitudinal RF electric field and a transverse focusing quadrupole field

  • Low-energy, high-current beams

  • Compact

  • Replacing Cockroft-Walton as injectors

2 MeV RFQ @ Idaho State Univ.

Accelerators and Ion Sources


Cyclotron

  • The cyclotron frequency of a non-relativistic particle is independent of the particle velocity:ω0 = eB0 / γm ≈ eB0 / m

  • E. O. Lawrence in 1929

  • Limitations: relativistic effects break the isochronism → Epmax≈ 12 MeV

Accelerators and Ion Sources


Isochronous Cyclotron

  • In order to restore the isochronism, the magnetic field needs to be shaped in function of the radius to match the change of the frequency with the particle energy.

  • However, such configuration leads to vertical orbit instability → restoration of the orbit stability using the Azimuthal Varying Field (AVF) L. H. Thomas (1938)

Accelerators and Ion Sources


Synchrocyclotron

  • Instead of modifying the magnetic field, the radio frequency can be modulated → pulsed beams

  • Limit at ~1GeV

  • Example: SC in CERN (600 MeV)

Accelerators and Ion Sources


Synchrotron

  • Use of the phase-focusing principle in a circular orbit with a constant radius

  • RF and magnetic fields are tuned to synchronize the particle revolution frequency and confine its orbit.

  • Examples:

    • PS, SPS, LHC @ CERN (28, 450, 7000 GeV)

    • SIS @ GSI

Accelerators and Ion Sources


CERN Accelerator Complex

Accelerators and Ion Sources


GSI The Present and the Future

Accelerators and Ion Sources


Ion Sources


Ion Sources

  • Very broad field with many applications:

    • Material science and technology (e.g. ion implantation)

    • Food sterilization

    • Medical applications

    • Military applications

    • Accelerators

    • ...

  • Beams of nanoamperes to hundreds of amperes

  • Very thin to very broad beams (μm2 to m2)

Accelerators and Ion Sources


Types of Ion Sources (selection)

source: http://linac2.home.cern.ch/linac2/seminar/seminar.htm#intro

Accelerators and Ion Sources


Plasma ion sources

  • Ionization is actually a process of creation of a plasma

  • Plasma ion source: Ionization mechanism: eˉ-eˉ collisions

  • Most widely used – many different types

  • Types differ according to plasma production and confinement mechanisms.

Accelerators and Ion Sources


Metal Vapor Vacuum IS (MEVVA)

  • Electrostatic discharge between a cold anode and a hot cathode in a vacuum

  • Evaporation and ionization of cathode atoms

Accelerators and Ion Sources


Penning Ion Sources

  • Arc discharge in a magnetic field – electrons confined radially by the magnetic field and axially by electrostatic potential well

  • In cyclotrons it is possible to use the magnetic field of the accelerator

  • One PIG is used @ GSI

Penning Ion Gauge (PIG) Ion Source

Accelerators and Ion Sources


Multi-Cusp Ion Source (MUCIS)

  • Cusp-like magnetic field lines

  • Most of the plasma volume in a relatively weak magnetic field

  • Large volume of uniform and dense plasma possible (2.5 cm – 1m size)

MUCIS used @ GSI

Accelerators and Ion Sources


Electron Cyclotron Resonance IS (ECRIS)

  • Vapor held in a cavity with high magnetic field

  • Microwaves with frequency that coincides with eˉ cyclotron frequency in the field heat the electrons (and only electrons).

  • No electrodes, no arc discharge – very reliable, high currents

  • 14 GHz, 0.5 T @ GSI, Dubna, LBNL, CERN

http://www.casetechnology.com/source.html

Accelerators and Ion Sources


Surface Ion Source

  • Hot surface of a metal with high work function ionizes elements with low ionization potential (like alkalis)

  • Negative surface ion source also in use

EXTRACTION ELECTRODE

Surface Ion-Source

http://isolde.web.cern.ch/ISOLDE/

Accelerators and Ion Sources


Sputter Ion Source

  • Cesium vapor, hot anode, cooled cathode

  • Some of the vapor gets condensed on the cathode, some gets ionized on the anode and accelerated towards the cathode where it sputters atoms from the cathode

  • Produces negative ions of all elements that form stable negative ions

Accelerators and Ion Sources


Laser Ion Source

  • Stepwise resonant excitation and photoionization of the atom

  • Chemically selective – wavelength tuned to the specific element

  • Pulsed

http://isolde.web.cern.ch/ISOLDE/

Accelerators and Ion Sources


Electron Sources

  • Thermionic emission – escape of electrons from a heated surface. Condition: Ee > φ

  • High field emission (fine point cathode)

  • Photo emission: λ < hc/φ

Accelerators and Ion Sources


The End

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